1. Field of the Invention
The present invention generally relates to data reproduction apparatuses for reproducing data recorded on a recording medium such as an optical disk, and more particularly to a data reproduction apparatus for reproducing data in accordance with maximum likelihood decoding from a signal reproduced from a recording medium on which the data is recorded in accordance with a recording signal with a partial response (PR) waveform.
2. Description of the Related Art
Conventionally, there has been proposed a recording and reproduction method using a PRML technique. According to this PRML technique, a recording signal obtained by modulating data to be recorded in accordance with a PR code is recorded on a recording medium such as a magneto-optical disk, a reproduced signal from the recording medium is sampled at a given frequency, and maximum likelihood (ML) data is reproduced from sampled values in a maximum likelihood data detector such as a Viterbi detector.
According to a data reproduction apparatus to which such a technique is applied, such as a magneto-optical disk unit, for instance, a signal is reproduced, or read out, from a recording medium recorded with a recording signal obtained by modulating data in accordance with PR(1, 1). Like the recording signal, the reproduced signal has its waveform equalized with a PR(1, 1) waveform as shown in FIG. 1A. Then, data having maximum likelihood as data before the PR(1, 1) modulation is reproduced from the transition states of strings of sampled values obtained by sampling the reproduced signal at a given frequency.
According to such a data reproduction apparatus, data is reproducible with good accuracy from a recording medium recorded with data at high density.
However, a reproduced signal from an MSR (Magnetically induced Super Resolution) medium that is a magneto-optical disk tends to have its level lowered slowly at a trailing edge part of the reproduced signal due to the characteristic of the MSR medium. Particularly, this tendency is noticeable if data is written to the MSR medium at a relatively high power. For instance, a reproduced signal corresponding to a 2T space part (a narrow distance part) between long marks (pits) formed on the medium by recording a signal modulated in accordance with PR(1, 1) may rise before lowering sufficiently as shown in FIG. 1B.
In the case of sampling such a reproduced signal having a characteristic that its signal level falls more slowly than rises in synchronism with, for instance, a clock signal synchronizing with rising edges of the reproduced signal, the transition of a sampled value Q is different from a correct one that should be obtained from a PR(1, 1) waveform at a trailing edge part of the reproduced signal indicated by a circle indicated by arrow A in FIG. 1C. Therefore, the recorded data is not faithfully reproduced by reproducing the data from the transition states of the sampled value Q by a maximum likelihood detector based on the premise of the PR(1, 1) waveform.